Investments - Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 20,938	$ 19,396
Gross Unrealized Gains	1,440	1,803
Gross Unrealized Losses	(32)	(11)
Fair Value	22,346	21,188
Allowance for credit losses recorded	0	0
Supporting experience-rated products
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	110	135
Gross Unrealized Losses	(1)	0
Fair Value	886	919
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,320	2,341
Gross Unrealized Gains	91	128
Gross Unrealized Losses	(1)	0
Fair Value	2,410	2,469
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,978	2,556
Gross Unrealized Gains	168	172
Gross Unrealized Losses	(2)	0
Fair Value	3,144	2,728
U.S. corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,754	7,879
Gross Unrealized Gains	822	1,023
Gross Unrealized Losses	(14)	(8)
Fair Value	9,562	8,894
Foreign securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,652	2,595
Gross Unrealized Gains	247	324
Gross Unrealized Losses	(6)	(1)
Fair Value	2,893	2,918
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	812	673
Gross Unrealized Gains	20	32
Gross Unrealized Losses	(3)	0
Fair Value	829	705
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,023	962
Gross Unrealized Gains	63	84
Gross Unrealized Losses	(5)	0
Fair Value	1,081	1,046
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,372	2,369
Gross Unrealized Gains	26	36
Gross Unrealized Losses	(1)	(2)
Fair Value	2,397	2,403
Redeemable preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27	21
Gross Unrealized Gains	3	4
Gross Unrealized Losses	0	0
Fair Value	$ 30	$ 25

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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